Resistivity tools are used in the oil and gas industry to determine the resistivity of earth formations surrounding a borehole. Conventional induction tools, for example, work by using a transmitting coil (transmitter) to set up an alternating magnetic field in the earth formations. This alternating magnetic field induces eddy currents in the formations. One or more receiving coils (receivers), disposed at a distance from the transmitter, detect the current flowing in the earth formation. The magnitudes of the received signals are proportional to the formation conductivity. Therefore, formation conductivities may be derived from the received signals.
However, the existence of a borehole complicates the derivation of formation conductivity from the received signals. The most prevalent complication that affects the derivation of formation conductivity from the received signals arises from the presence of drilling fluids in the borehole surrounding the induction instrument. This is referred to generally as the borehole effects. Often, the fluids in the borehole (drilling mud) are made very saline, thus conductive, as part of the drilling practice. The conductive drilling muds can contribute a significant proportion of the received signals and, therefore, should be carefully removed.
In addition, tool properties may affect the measurements conductivity tensor. The effects of the borehole and tool properties on the measured conductivity tensor may be very significant, even in a highly resistive, oil base mud (OBM) environment. Unless the borehole/tool effects are removed or otherwise compensated for, it is hard to use or interpret the measurements to infer formation properties.